THE BLAST FURNACE. 81
steain machinery, this surplus should be about 2000 indicated horse-power. With gas engines it should be 7000 horse-power. The above figures are true only for usual operating conditions, for with an unusually low fuel ratio there will be less surplus, while with abnormally high coke consumption the surplus will be greater, but the variation is less than might be expected, as the calculations are based on a ton of coke charged and not a ton of iron smelted.
SEC. IIw.—Composition of pig-iron.—Carbon: The metal produced by the blast furnace is not pure iron, for while it is in contact with white-hot coke it absorbs a certain proportion, of carbon. The amount absorbed is quite constant, seldom being less than 3.25, nor more than 4.25 per cent. When the iron is in a melted state all of this carbon is chemically combined with the iron, but as the metal cools there is a tendency for it to separate as graphite. This separation requires an appreciable time and can be prevented by sudden cooling. • If a small quantity of liquid iron be chilled in a stream of water or an iron mold, almost all the carbon will remain combined, and the metal be hard and brittle. If, on the other hand, a large mass of iron be poured in sand and covered so as to cool slowly, the separation of carbon will go on for a long time, and the resulting metal will be soft and tough and a fractured surface will, exhibit loose flakes of graphite.
Silicon: Pig-iron contains silicon from the reduction of silica; Si02-)-2C=:Si+2CO. This silica is always present in iron ore, in the ash of the coke and in the limestone. It is difficult to reduce, and if the temperature of the furnace is low the iron will contain only about one-half of one per cent, of silicon, while if the furnace is hot the reducing action of the coke is more powerful and the iron may contain four or five per cent.; while under special conditions an alloy called ferro-silicon may be produced with over ten per cent. Silicon tends to drive carbon out of combination into the free or graphitic state, so that a pig rich in silicon will usually have an open fracture, but this iron will often contain less carbon than ordinary iron, as the high silicon prevents the absorption of the usual proportion.
Phosphorus: The amount of phosphorus present in pig-iron depends upon the materials used, for whatever of this element exists in the ore, in the coke, or in the limestone will be found in the metal. In pig-iron intended for foundry work the phosphorus may